loader: Allow dependencies to use ":" to refer to junctioned elements

• loader: Allow dependencies to use ":" to refer to junctioned elements

  In the previous commit, we added support to express cross-junction dependencies inline as simple strings. Document it along with the version in which the feature was added.

  This will allow cross-junction dependencies to be listed as strings on a single line.

  As part of this, some logic around initializing Dependency() objects have been moved out of _extract_depends_from_node() method into the constructor of Dependency() class, to keep all related code in one place.

  • _loader/types.py: While initializing Dependency objects, attempt to split filenames, only if no junction was specified explicitly. If a junction was specified, then filenames with : in their names will result in an error.

  • _loader/loadelement.py: Refactor logic to initialize Dependency() objects to move it to the Dependency() constructor.

  • tests/frontend/buildcheckout.py: Add tests to ensure the above.

  • _versions.py: Bump BST_FORMAT_VERSION.

• NEWS: Cross-junction dependencies can now be specified inline

• Document how to refer to cross-junction dependencies inline

  In the previous commit, we added support to express cross-junction dependencies inline as simple strings. Document it along with the version in which the feature was added.

Fixes #809 (closed).